Getting the Most From Fisher FM Stereo MPX Tuners and Receivers

All of the transmitters in the Wash/Balt/Annapolis area are at least 25-30 miles from me and I'm on a peninsula oriented NW-SE. Towers are NW, N and SW of me, with my mostly listened to stations 45-60 miles away. The area I'm in is at sea level (<less than 100Ft ASL)and is a notoriously lousy area for FM Stereo. FM MONO is not really a problem. The closest station with a 1/2 decent transmission is WLIF (101.9) which gives me a 3 of 5 on any receiver I use. The tower (1500 ft) is approx 23 miles away. The majority of the stations show between 1.5 to 2.5 and vary during the day and night. So any unit with a Stereo Beam Dances in FM usually with the low frequency beat. Some more than others depending on how well or poorly they are aligned since the factory alignment.

Revisiting this great old thread hoping to find the answer to three issues. These are on my late model 400 receiver.
1. The dancing eye tube mentioned here. Was it addressed in another thread, and if so can someone point me?
2. The 2 bars on my tube are not symmetric - one side is about 1/16" longer than the other all the time. Suggestions?
3. How the #@%$& do I get the rubber boot of the eye tube?

1. I don't believe Dave ever addressed it as several posts ago I asked if there was any cure for the dance.'
2. It sounds like your eye tube needs to be centered on the clips that hold it in. My 400 was that way when I bought it.
3. Carefully working it off the tube pins. They are fixed on there much tighter than any other tubes.

1.) Dancing tube was adressed in one of the 400 specific or MPX threads. See if I can find it.
2.) Grab the tube and slide it slightly in the clips, to the side that has more bar.
3.) Streetwise said it. a little silicon lube on the wires will help it slide down after it comes off the socket lip.

EDIT for #1. I checked all the 400 and mpx threads Dave started. only passing mention of the dancing eye tube, and has NOT been addressed to fix or mod, that I can find.

It took a lot of force to free the tube from the clips, and now the tube won't light at all. Hopefully just loosened a wire. It is an original Fisher-badged EM-84A tube. I stuck a EM-84 (non-A) in the socket & it did not light up either so don't think it is the tube. Fun, fun, fun.

Larry, cleaning the socket & the pins on the tube didn't help. Just for sanity check, I checked the voltages & got the following. These were with the tuner tuned to a fairly strong local station, in FM mono mode. Pin 1 was 167 v, 2 was 0, 3 was 167, 4 was 147.7, 5 was 6.4 vac, 6 was 0, 7 was 0, 8 was 0, 9 - between -1.75 and -1.6. Do these look generally ok? If so, I will start looking at a replacement tube.

Larry, cleaning the socket & the pins on the tube didn't help. Just for sanity check, I checked the voltages & got the following. These were with the tuner tuned to a fairly strong local station, in FM mono mode. Pin 1 was 167 v, 2 was 0, 3 was 167, 4 was 147.7, 5 was 6.4 vac, 6 was 0, 7 was 0, 8 was 0, 9 - between -1.75 and -1.6. Do these look generally ok? If so, I will start looking at a replacement tube.

Pio, can you explain further? It worked fine (except for dancing) until I tried to slide it in the clips. It does not look like any of the wires pulled loose. The tube does not light up at all - no glow from the heater, nothing.

He means check for one, check the heater circuit lines pins 4 & 5 are your heater. Set your meter to ACV and put a probe on each of pin 4 & pin 5 and post results. You need to start learning the pinout's on the various tubes (at least the heaters or filaments).

Sorry, I did not understand that was what he meant. I have a have a basic idea on pentodes & triodes, this sort of tube is all new ground for me. As noted above I get 6.4 VAC pin 5 to ground, same between pin 4 & 5 (heater circuit). Since I see no glow at all from the tube, I think I broke something inside. The other voltages do not match this diagram I pulled off radiomuseum.org, but I don't really understand how the tuning would affect this. I was guessing that pin 1 voltage would vary as stations were tuned. That was why I was asking for a sanity check on the voltages I was seeing (especially the DC voltage on pin 4 of the heater circuit).

Attached Files:

Sorry about delay regarding this modification. I've just had too many irons and not enough fire lately! But it seems that there is interest in this issue after all. In the 500C and 800C receivers and 100-B, 200-B, and R-200 tuners, this issue also manifests itself as a low sensitivity (or high threshold) for triggering the unit into FM Stereo mode automatically. As a result, most pics you see of one of these units operating with the Stereo Beacon lamp illuminated will almost always show the selector switch in the FM Stereo Filter position -- as that position creates a forced stereo reception mode, turning on the Stereo Beacon lamp at all times. That is, all stations will pass their signal through the MPX section whether they contain the identifying 19 kHz pilot signal or not. Even if no station is tuned in, the lamp will be lit. Theoretically, this should not be a problem but in reality it is, as purely mono stations (i.e. talk radio, etc) will typically display a slight distortion if listening to them through the MPX circuitry (i.e., in stereo mode). This in fact is the reason for providing the ability to switch an FM Stereo MPX tuner back and forth between stereo and mono modes -- either manually as done with the 400, or automatically as done in the Stereo Beacon models. In FM Mono mode, the audio signal is taken directly from the Ratio Detector stage, bypassing the MPX circuitry. That way, any unbalanced signals from the free running 38 kHz oscillator on the MPX sub-chassis can't find their way into any purely mono FM audio, and cause the distortion otherwise to be generated.

OK. So the problem at hand then is, if you're going to provide some sort of FM Stereo MPX indicator, what should the threshold be for the unit to trip modes into or indicate stereo reception? And why have any threshold at all? Well first, there is the issue just discussed above. There needs to be some preset threshold level so that the unit can have a knowledge of when to indicate or automatically trigger -- or not, just the same way a thermostat in your home triggers the heat or AC systems. But the threshold itself (i.e., where the thermostat is set) can be a matter of discussion.

As designed, the threshold was set rather high. To understand why, you need to understand the realities of the day: It's 1963. FM Stereo has just been approved by the FCC. Of the two or three FM stations that existed in an average size city, maybe only one or two were stereo, and at that, not all the time. The crazy crowded FM band of today with stations belching out 100% modulation 99.9% of the time at seemingly every .2 mHz interval on the dial could not have even been imagined, let alone realized. And, the programming was almost guaranteed to be classical or light jazz, and that was IT. Importantly, processing of the audio signal was minimal. So what's the difference between these two scenarios? NOISE! Not the music (although much of it is), but quite literally, noise, as in noise from the 38 kHz AM sub-carrier that comprises the stereo information sub-channel within the main FM carrier signal. If reception was not strong, and there were numerous quiet spots in the classical music format, then any noise was going to be very apparent -- and we can't have that when showcasing the brand new FM Stereo MPX baby now can we! What to do? Set the stereo switching threshold very high, and get the word out every way possible about how much better your antenna must be to receive the new FM MPX Stereo signals properly. That way, when the Stereo Beacon is triggered, its guaranteed to be a huge selling point and sound great.

In the ensuing nearly 55 years however, things have changed: The Effective Radiated Power of FM stations is typically far greater than those of the early pioneering stereo stations. Signal processing has elevated average modulation levels much higher. Recording processes have improved, and tastes in music have shifted. The bottom line, is that usable FM MPX Stereo signals are available today at weaker carrier levels than was the norm in the early 60s. Against this, the old threshold standard that Fisher used is obsolete. Today, that threshold can be lowered for more dependable Stereo Beacon switching, and less tango with the 400's eye tube.

So where exactly is the threshold for stereo switching established in your favorite Fisher MPX unit? To keep the discussion simple, we'll limit it to the MPX-65, and its close cousin, the WX chassis. There are some other close sub-chassis kin as well, but this will cover the vast majority of pieces out there.

In the classic Fisher multiplex circuit (nearly standardized and available with the schematic in the service manual of every unit identified above), there is hidden away in the circuitry of V100 a noise detector circuit, comprised of L101, CR100, R204, and C207. The Cliff notes version of how it operates is that Z-100 is part of a tuned circuit that is peaked at 19 kHz, allowing maximum transfer of the 19 kHz pilot signal that is part of the composite signal appearing at plate of the first section of V100 (pin 1), into the grid of the second section of V100 (pin 7). Any noise components of the signal at pin 1 are rejected by the tuned circuit of Z100, and therefore appear across L101. CR100 rectifies any voltage appearing across L101, with its polarity set to produce a negative DC voltage at its output. This voltage is filtered by R 204 and C207, and applied to the pin 7 grid via R203. As designed then, the more noise voltage that is developed at the output of CR100, the greater the negative voltage is that is applied to the grid at pin 7. This voltage can easily reach levels great enough to "cut off" (or nearly so) the second section of V100, which then greatly limits the triggering signal sent to V101, and the indicator output. Hello dancing eye tube, and Stereo Beacon that rarely triggers.

The answer to limiting this noise detector action is really rather simple, and consists of adding a single quarter watt resistor to the circuit, based on the tuner that the MPX chassis is being used with. For all units, the resistor is installed across R204 (i.e., from the anode of CR100 to ground), with its value roughly based on the number of IF limiter tubes used in the tuner section. Therefore:

1. All receivers should have a 47K resistor installed.
2. FM-100B and 200B tuners should have a 68K resistor installed.

The values indicated are purely of my own determination for my own use. I have installed these values in my 400 and 800C receivers, and 100-B, 200-B, and R-200 tuners, and they all work fine. Of course, they are based on properly aligned and operating units. Using higher values raises the threshold, while lower the values will reduce the threshold even lower -- but watch out that you don't go so low as to allow false indicator triggering. The values chosen steer safely clear of that issue in my units, and generally allow the lowest practical threshold to be had in normal use. The bottom line is that now in FM Automatic, instead of the Stereo Beacon indicating when the old stereo threshold has been met, it now simply indicates reception of a stereo station, and I can then tell the unit if/when I chose NOT to listen in stereo by turning the selector switch to mono FM. That approach just works better for me, as I rarely if ever turn my units to FM mono. They just remain in FM Automatic virtually all of the time now, with the Beacon triggering off and on between stations, and between mono and stereo stations as you would expect. On my 400, the eye tube is now well behaved on all but the very weakest (i.e. unusable) stereo stations.

[/QUOTE]In the classic Fisher multiplex circuit (nearly standardized and available with the schematic in the service manual of every unit identified above), there is hidden away in the circuitry of V100 a noise detector circuit, comprised of L101, CR100, R204, and C207. The Cliff notes version of how it operates is that Z-100 is part of a tuned circuit that is peaked at 19 kHz, allowing maximum transfer of the 19 kHz pilot signal that is part of the composite signal appearing at plate of the first section of V100 (pin 1), into the grid of the second section of V100 (pin 7). Any noise components of the signal at pin 1 are rejected by the tuned circuit of Z100, and therefore appear across L101. CR100 rectifies any voltage appearing across L101, with its polarity set to produce a negative DC voltage at its output. This voltage is filtered by R 204 and C207, and applied to the pin 7 grid via R203. As designed then, the more noise voltage that is developed at the output of CR100, the greater the negative voltage is that is applied to the grid at pin 7. This voltage can easily reach levels great enough to "cut off" (or nearly so) the second section of V100, which then greatly limits the triggering signal sent to V101, and the indicator output. Hello dancing eye tube, and Stereo Beacon that rarely triggers.

The answer to limiting this noise detector action is really rather simple, and consists of adding a single quarter watt resistor to the circuit, based on the tuner that the MPX chassis is being used with. For all units, the resistor is installed across R204 (i.e., from the anode of CR100 to ground), with its value roughly based on the number of IF limiter tubes used in the tuner section. Therefore:

1. All receivers should have a 47K resistor installed.
2. FM-100B and 200B tuners should have a 68K resistor installed.
Dave[/QUOTE]

Dave, this reminds me of several exchanges we had when I was restoring my low SN 500C. I had the poor stereo switching action on FM Auto. Sometimes I could switch to the FM Stereo position and then back and it would stay switched and sometimes not. Other times it might switch automatically and then drop out later due to slight signal level fluctuations. I even tried some alternate values bridged across R204 at the time, but took them out thinking that it should work as originally designed. When I was servicing these at the dealer I worked for when these units were new and in warranty, it was in a metropolitan area close to the transmitters. When I was working on my 500C and air testing it here, it is in a fringe area and using an outside antenna to get a reasonably decent signal level. I had looked at that circuit and realized that it was biasing the second stage of V100 to limit the effects of noise and limit spurious switching. I think I will pull the 500C out once more and give this a try. I suspect it will solve at least some of the issues I was dealing with at the time.

Larry, the 47K should be fine, as the overall gain/limiting action of the two versions are pretty similar, with the early version using a higher Gm tube at the input of the three tube IF strip to accomplish what the later version did with four tubes of less Gm. I don't have a 3 tuber IF unit here to test it on, but it should will work fine. If you have an early version and can test its operation with the resistor installed, please let us know how it performs!

I've got one on my bench, just waiting for me to finish it up. Now that Dorcas is back home, I should be able to get her to keep the dog occupied, instead of underfoot while I've got fingers near live circuits. I'll do both and give you an A/B.